Selenium is an essential element, but becomes very toxic at higher concentrations. It occurs in the environment at concentrations ranging from nM to uM and selenium pollution is a worldwide phenomenon. This works aims at improving the knowledge on the interactions between selenite - Se(IV) - and a freshwater phytoplanktonic organism: the unicellular green algae Chlamydomonas reinhardtii. The aim of the performed experiments were: i) to investigate selenite -Se(IV°)- uptake mechanisms in C. reinhardtii, using Se75 as a tracer in short term exposures (<1h); ii) to assess selenite toxicity as measured with growth impairment and ultrastructural damage (with EDAX-TEM analysis), using long term exposures (96h) to stable selenite; iii) to evaluate the bioaccumulation capacity of selenite and its potential links with toxicity.Short-term experiments revealed a negligible adsorption and a time-dependent linear absorption with an estimated absorbed flux of about 0.2 nmol.m-2.nM-1.h-1. The uptake was proportional to ambient levels in a broad range of intermediate concentrations (from nM to uM). However, fluxes were higher at very low concentrations (< nM), and decrease with increasing high concentrations ( > uM), suggesting that a high affinity but rapidly saturated transport mechanism could be used at low concentrations, in parallel with a low affinity mechanism that would only saturate at high concentrations (~mM). The latter could involve transporters used by sulphate and nitrates, as suggested by the inhibition of selenite uptake by those element. Se(IV) speciation changes with pH did not induce significant effect on bioavailability.On the basis of the relationship between Se concentration and maximal cell density achieved, an EC50 of 80 uM ([64; 98]) was derived. No adaptation mechanism were observed as the same the same toxicity was quantified for Se-pre-exposed algae. Observations by TEM suggested chloroplasts as the first target of selenite cytotoxicity, with effects on the stroma, thylakoids and pyrenoids. At higher concentrations, we could observe an increae in the number and volume of starch grains. For the cell collected at 96 h, electron-dense granules were observed. Energy-dispersive X-ray microanalysis revealed that they contained selenium and were also rich in calcium and phosphorus.Finally, growth inhibition was highly correlated to the bioaccumulation of selenite. The latter was inhibited by increasing concentrations of sulphate, and calculated dry weight concentration factors varied between 690 and 7100.